Page 130 - HVAC Pump Handbook

Rishel_CH05.qxd 21/4/06 6:12 PM Page 127

Physical Description of HVAC Pumps

Physical Description of HVAC Pumps 127

5.7.1 Forces on centrifugal pumps
The forces exerted on a volute-type pump are: (1) axial and (2) radial,
as shown in Fig. 5.15 for a volute-type pump. The axial thrust is com-
puted by the pump designer who provides the correct type of thrust
bearing to withstand the imbalance of radial thrusts that exist in
such a pump. Figure 5.17 illustrates these forces for both a single and
a double-suction pump.
The axial forces for a double-suction pump appear to be balanced,
and such is the case for a perfect design and installation of this type
of pump. This assumes that the flows into each of the suctions are
equal. In actual practice, this may not be true due to uneven machin-
ing of the suction passages of the impeller or by not having the
impeller centered in the casing. Also, improper configuration of the
suction piping may cause the flow into the two suctions to be unequal.
The pump designer realizes this can happen and provides a thrust
bearing that will withstand the thrust of some unequal flow. Since the
direction of thrust can be either way, the bearings must be designed
for the force in both directions. This is unlike the single-suction pump
where the axial thrust is almost always toward the pump suction.
The single-suction impeller for the volute pump does have unequal
axial thrusts since the suction pressure exerts less force on the
impeller than that generated by the higher discharge pressure. The
thrust bearing for this pump must be designed to accommodate this
difference. It is obvious that single-suction, volute pumps designed for
high heads must have thrust bearings capable of handling such
higher differences in radial thrusts.
When axial-flow pumps are installed vertically, the axial thrusts are
named “downthrust” or “upthrust.” Due to the weight of the rotating

Figure 5.17 Axial thrust in volute-type pumps. (From Karassik et al., Pump Handbook,
3d ed., McGraw-Hill, used with permission.)

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.